It is known that when a driver is to send a digital signal to a remote receiver through a transmission line there can be problems in the case in which there is no good match between the transmission line impedance and the input and output impedances of the circuits connected therewith. Generally, in these cases there are power losses and multiple reflections which can cause an error probability increase in the symbol transmitted at a given instant, because, in addition to the symbol transmitted at the given instant, there are present at the receiver, vestiges of symbols transmitted in preceding time intervals and reflected by the line ends.
These disadvantages can be eliminated if at least the driver output impedance is rendered equal to the characteristic impedance of the line. In fact, even though there may be a mismatch at the end of the line connected to the receiver, the input impedance of a rather long line is almost equal to the characteristic line impedance. That would result in a reflection at the far end of the line, but the reflected signal is absorbed at the input by the driver output impedance, whose value is equal to the line impedance, thus avoiding multiple reflections.
However, drivers with output impedance equal to the characteristic impedance of the usual transmission lines are not easy to build, owing to unavoidable tolerances in the fabricating process. Transmission lines can have tolerances in the characteristic impedance also, but these tolerances are typically much smaller than those of integrated circuits. It is hence advisable to have an automatic system for adjusting the driver output impedance so as to obtain a good match to the line, independent of fabrication tolerances and accidental circuit variations.
It is also convenient that the driver output impedance be made equal to the characteristic line impedance without using an external additional resistance, in order to avoid an oversizing of the output circuit with consequent speed loss and power consumption.
A solution to these problems has been described in the article entitled "A Self-Terminating Low-Voltage Swing CMOS Output Driver" by Thomas F. Knight et al., IEEE Journal of Solid State Circuits, Vol. 23, No. 2, April 1988.
In this system, the output impedance of a plurality of drivers contained in the same integrated circuit is controlled by a suitable circuit, also housed in the integrated circuit, comprising a reference driver, analogous to those controlled, and a receiver. The reference driver sends a clock signal, locally-generated for this control purpose, onto a transmission line length with the same characteristics as those of the lines connected to controlled-driver outputs. The line output is connected to the input of a receiver that extracts from the received signal an enabling signal synchronized to logic-level transitions.
The voltage at the input of the reference line is read continuously by a threshold comparator, whereby it is compared with a reference voltage of value equal to half the maximum value of the output voltage of the driver under matching conditions. The information at the output of the comparator is used to control the driver output impedance.
However, this circuit has a number of disadvantages.
A first disadvantage is the comparison of the near-end line voltage in lieu of comparing the far-end line voltage to a reference. This requires additional circuitry for exact timing of the comparison, which has not been disclosed in the paper.
The second disadvantage is the requirement that the (0.5 V) reference voltage is required to be precisely equal to one half of the (1 V) supply voltage of the last driver stage and that any deviations from this ratio produces a corresponding mismatch condition. Unavoidable tolerances in integrated circuits make this precise ratio disproportionately costly to achieve.
The third disadvantage is the use of additive 1 V, -5 V and 0.5 V supplies, which is not economical for integrated circuit manufacture.
Finally, the circuitry for proper timing of the comparator is not provided.
Another solution to these problems has been described in the U.S. Pat. No. 5,095,231 entitled "Automatic System for Adjusting the Output Impedance of Fast CMOS Drivers" by Sartori et al.
This system uses the same principles as Knight and Krymm, but also has several disadvantages, and also compares the voltage at the input of the reference line to a reference voltage.
A first disadvantage is that it uses an off-chip resistor for termination of the transmission line. Consequently, the driver on-resistance has to be negligible compared to the characteristic line impedance, leading to an oversized output circuit with speed loss and greater power consumption. Furthermore, the external termination resistor drastically increases the manufacturing costs.
A second disadvantage is due to the additional 0.5 V reference supply required for comparison of the near-end line voltage and the requirement that this supply voltage precisely equals one half of the supply voltage of the last driver stage. This is not practical for the manufacture of integrated circuits.